1. Field of the Invention
The present invention relates to the design of an attenuation reduction structure for high-frequency signal transmission lines of a circuit board, and in particular to the formation of attenuation reduction patterns at locations corresponding to bottom angle structures and top angle structures for high-frequency signal transmission lines of a flexible circuit board.
2. The Related Arts
Most electronic devices are provided with circuit boards or flexible flat cables to position all necessary circuit components and connectors and realize transmission of electronic signals. In the manufacture of a circuit board, wires are laid on a surface of a substrate to form an extended signal transmission line for transmission of the electronic signals.
Referring to FIGS. 1 and 2, which are respectively a cross-sectional view and a partial enlarged view of a conventional flexible circuit board, as shown in the drawings, a substrate 1 comprises a first surface 11 and a second surface 12. On the first surface 11 of the substrate 1, a plurality of parallel high-frequency signal transmission lines 21. 22 is formed and spaced from each other by a predetermined spacing distance d1. The high-frequency signal transmission lines 21, 22 are arranged in pair, generally in combination with an adjacent grounding line G, to transmit at least one differential-mode signal. A covering insulation layer 3 is formed on the first surface 11 of the substrate 1 and covers surfaces of the high-frequency signal transmission lines 2 and the grounding line G.
The high-frequency signal transmission lines 2 are generally made of a copper foil material or a composite material and are of a structure having a cross-section of a substantially flat configuration. Ideally, the high-frequency signal transmission lines each have opposite sidewalls that are vertical walls. However, in a real structure, the two sidewalls of a high-frequency signal transmission line show a slope shift amount so as to each form a top angle structure. For example, the high-frequency signal transmission line 21 has two sidewalls that are supposed to be normal to the first surface 11 of the substrate 1. However, in a real structure, the two sidewalls of the high-frequency signal transmission line 21 often show a slope shift amount d2 (namely being non-vertical sidewalls), so that a left bottom corner and a right bottom corner of the high-frequency signal transmission line 2 each form a bottom angle structure 21a, 21b and a left top corner and a right top corner of the high-frequency signal transmission line 21 each form a top angle structure 21c, 21d. In other words, the high-frequency signal transmission line 21 has a bottom width w1 that is greater than a top width w2.
In the transmission of a low-frequency signal, the angle structures of the high-frequency signal transmission line does not cause any problem. However, in the transmission of a high-frequency signal, the top angle structures and the bottom angle structures may cause attenuation of the high-frequency signal. Such a situation leads to problems of poor signal transmission reliability, poor impedance control, and signal interference in the transmission of a high-frequency signal through high-frequency signal transmission lines that are commonly used in the contemporarily available electronic devices.